The process of solidification of liquid CO.sub.2 to form solid CO.sub.2, commonly called "dry ice", is a widely known process, with the product being used commercially to transport and store medicines or already frozen or fresh foods, to cause artificial rain, to extinguish fire, to contract metal, etc.
Typically, the conventional processes for producing solid CO.sub.2 involve first creating a mass of CO.sub.2 snow at a pressure near atmospheric pressure, and then compressing this mass, giving it a definite form and specific density. The average capacity of the conventional measurements (25.4 cm.times.25.4 cm.times.25.4 cm) is approximately 1 cube/5-6 min. or more and the equipment consists basically of: a) closed chambers designed to support the conditions inherent in the formation of carbon dioxide snow; b) gas recovery equipment; c) hydraulic compression systems; d) CO.sub.2 storage tanks to which the gas returns, and e) condensation systems.
This equipment operates fundamentally by expanding liquid CO.sub.2 to a pressure near 5 kg/cm.sup.2 to create a mass of carbon dioxide snow from which gas is extracted via a compression element, to thus be able to reach a pressure close to atmospheric pressure, and later compressing this mass at 125 kg/cm.sup.2 in a mold by means of a hydraulic system so that the resulting compressed mass of dry ice is later expelled with a maximum density of 1.55 g/cc.sup.3. Because of the relatively high pressures required to compress the carbon dioxide snow to form a block of relatively high density, the equipment required is expensive to build and to operate.
Other processes that differ from the above, already traditionally used, have been disclosed in many patents, which are generally based on the thermodynamic properties of CO.sub.2.
The concepts and details of the known processes and equipment can be found disclosed in the following U.S. Pat. Nos. which are included here for reference: 1,925,041; 1,925,619; 1,870,691; 1,950,180; 1,795,772; 2,738,658; 4,033,736; 3,817,045 and 4,780,119.
U.S. Pat. No. 1,925,041 to Auerbach describes a two-stage process where initially a layer of solid CO.sub.2 is created in the bottom of a mold that serves as a seal and support, which is sufficiently dense to prevent the passage of gaseous CO.sub.2 across it, after which liquid CO.sub.2 is injected at a high pressure of 20 atmospheres (21.9 kg/cm.sup.2). The liquid CO.sub.2 evaporates slowly to form a dense block of CO.sub.2 with a specific gravity above 1.4.
On the other hand, the U.S. Pat. No. 1,925,619 to Zumbra is concerned with the filling of a cylindrical chamber with CO.sub.2 snow at atmospheric pressure and then injecting liquid CO.sub.2 into the snow to create a mass of CO.sub.2 snow that is saturated with cold liquid CO.sub.2 under high pressure (above the triple point), for example, 5.47 kg/cm.sup.2, in which form it is caused to solidify by freezing by means of filling an annular surrounding chamber with CO.sub.2 snow at -77.2.degree. C.
The U.S. Pat. No. 1,870,691 to Rust et al. shows an extrusion device modified to produce solid CO.sub.2 that uses an intermediate zone in which a mass of CO.sub.2 snow is saturated with liquid CO.sub.2 at 21.9 kg/cm.sup.2 and is left to evaporate slowly. In general, the apparatus of the Rust et al. patent is similar to that disclosed in U.S. Pat. No. 1,925,041 to Auerbach, discussed above.
U.S. Pat. No. 1,950,180 to Jones et al. discloses several devices for producing blocks of solid CO.sub.2 that have a generally uniform density above 1.5 g/cm.sup.3. It discloses the concept of filling a chamber with snow at a pressure around atmospheric pressure and then flooding the chamber with liquid CO.sub.2 at a pressure above the triple point to saturate and submerge the snow in the liquid and later slowly vaporize the liquid at a pressure very close to the triple point in order to create average large size CO.sub.2 crystals, which are then compressed to form high density solid CO.sub.2.
U.S. Pat. No. 1,795,772 to Goosman is directed generally to a compression and extrusion process, and U.S. Pat. No. 2,738,658 to Bronson shows the production of snow fed to a conventional dry ice pressure.
U.S. Pat. No. 4,033,736 to Cann is directed to a method and apparatus for making bars or spheres using a compression/extruder chamber and applying rotary force against a pressure applied by a spring.
Based on the disclosures of the above discussed patents, it is apparent that some of the processes of the prior art are designed for production of cubes of solid CO.sub.2 by compression to product densities of 1.4-1.5 g/cm.sup.3, and that other equipment produces cubes of solid CO.sub.2 with average densities below 0.6-0.8 g/cm.sup.3. The lower density form of carbon dioxide cannot satisfy the needs of the industry for dry ice, and its use is restricted to less demanding applications.
From the previously discussed patents, we can summarize that the invention disclosed in U.S. Pat. No. 1,925,041 is of particular interest, as it involves a process without compression which operates at a pressure above the triple point of the liquid CO.sub.2 that passes instantaneously into a chamber, forming a layer of snow for sealing purposes and then the chamber is filled with liquid CO.sub.2. The '041 patent involves a process in which the liquid CO.sub.2 is introduced at substantially higher pressure than in the process of the present invention. The rate of CO.sub.2 solidification in the '041 patent would be very slow in view of the lack of any effective CO.sub.2 gas venting from the mold during solidification. The '041 patent does not include the CO.sub.2 gas recovery system or the concept of feeding the liquid CO.sub.2 in two stages as in the method and apparatus of the present invention.
All the remaining patents describe sundry methods for producing solid carbon dioxide and are of little relevance with respect to the method and apparatus of the present invention.